Dot matrix printers use a printing head which includes a vertical row of seven to eleven fine wires guided by holes drilled in a block and driven selectively to strike a film or cloth ribbon as the printing head moves across a piece of paper. Individual characters are formed by selectively printing a matrix of dots using the fine wires or printing elements. The matrix is typically from seven dots wide by seven dots high, up to seven dots wide by eleven dots high. The more dots used, the better will be the quality of the characters that are printed. This type of printer is in common use to produce printouts of computers and word processors where the speed of conventional type printers is inadequate. This type of printer is substantially faster than type printers, but not so fast as line printers which can print entire lines of type simultaneously.
Conventional dot matrix printers may be operated magnetically using a magnetic relay in which a coil or solenoid moves an armature that drives one of the printing wires or elements. One such relay is required for each of the seven to eleven wires; the relays may be arranged in a circle around the block which guides the wires to the print wires. The relay devices are made as small as possible to permit operation at high speed, and also to minimize obstruction of the operator's view of the paper being printed. However, such magnetic drivers are not very energy efficient because of the resistance heating in the coils, and the efficiency decreases as the coils are made smaller. Thus the heating problem limits the speed of operation of the driver relays or solenoids of the matrix printers, although they do reach a speed of over 200 characters per second.
Another form of magnetically operated dot matrix printer uses a spring to drive each printer wire or element toward the paper. Each printer wire is retained in a cocked position by a permanent magnet acting against an iron head attached to each of the print wires. The print wire is released by the energization of a solenoid which provides an opposing field to that of the retaining permanent magnet. Energy is stored in the spring by the permanent magnet prior to the printing release. These types of stored-energy printers tend to be more expensive and are still not significantly faster than the armature-driven printers. Piezoelectric devices are inherently more energy efficient and involve less moving mass than magnetic devices, but attempts to design a piezoelectric printing head have not been too successful. Piezoelectric materials expand in an electric field if one polarity is applied to them and contract when that polarity is reversed. They produce very high forces but very small displacements. To increase the displacement, one approach is to stack small piezoelectric disks, each energized by its own voltage source, electrically in parallel but mechanically in series, known as stack motors. These motors do generate enough displacement to drive a print wire with sufficient energy to make a dot impression, but their mass is too large to operate any faster than the equivalent magnetic armatures, and their cost can be much higher. Bilams are a common piezoelectric component made of two piezoelectric members laminated together, but they typically require large volume to generate the necessary energy for impact printing and this in turn requires high drive voltage, which is hazardous and is difficult to provide.
Speed, small size and compactness are sought in all of these designs with the purpose that if sufficient speed could be attained, two or more impacts could be made for each character, thus making it possible to compact the printer elements and to overlap individual dots in a staggered matrix array so that continuous lines rather than dotted lines could be used to print the characters and thereby improve the print quality. Or alternatively, the speed could be increased while the quality remains unchanged.